(a) Field of the Invention
The present invention relates to a treating liquid for offset masters, and particularly it relates to a treating liquid for the purpose of desensitizing an electrophotographic offset master having a photosensitive layer consisting essentially of fine zinc oxide powder and a binder in order that said offset master can be used as an offset printing plate.
(B) Description of the Prior Art
An electrophotographic offset master having a photosensitive layer consisting essentially of fine zinc oxide powder as photoconductive substance and a binder is generally prepared by the procedure comprising preparing an electrophotographic copying material by forming a photoconductive layer wherein fine zinc oxide powder, an insulating resin as the binder and, when necessary, sensitizing coloring matters, etc. are uniformly mixed together and coated on a support, forming a desired image on the photoconductive layer side of this copying material through the known steps, to wit, electrification, exposure and development, and thereafter treating it with a desensitizing liquid (hereinafter called `treating liquid`).
A treating liquid for this purpose desirably is capable of imparting a sufficient aptitude for offset printing to the offset master; for instance, it should have an intense desensitizability and be capable of facilitating the adhesion of printing ink onto the image area.
There have already been developed treating liquids for offset masters which can satisfy the foregoing requirements in some measure and such liquids have been put to practical use. To cite an example of a currently utilized treating liquid, there is one which consists essentially of ferrocyanate and water-soluble phosphate such as disclosed in Japanese Pat. Nos. 33683/1970, 21244/1971, etc. These ingredients are said to have the effect of making the non-image area of the offset master hydrophilic more rapidly and increasing the strength of the hydrophilic coating film formed on the nonimage area of the photoconductive layer. U.S. Pat. Nos. 3,522,062 and 3,769,043 disclose treating liquids which are intended to increase the strength of the hydrophilic coating film by adding a hydrophilic high molecular substance such as sodium alginate, polyacrylic acid or the like to said basic composition.
However, as a matter of fact, it has so far been impossible to prepare a satisfactory offset master by the use of the conventional treating liquids such as cited above.
That is to say, in order to obtain prints by the use of an offset master prepared as above, it is satisfactory to follow the procedure comprising setting the offset master around the plate holding cylinder of an offset printing machine, applying a 1 in 6-10 solution of treating liquid (to wit, wetting solution) all over the surface of the thus set offset master, inking over the offset master so as to make the printing ink adhere selectively to the image area thereof, transferring the thus applied printing ink onto a blanket, and transferring the printing ink held on the blanket onto a transfer paper consisting of ordinary paper. On this occasion, because an offset printing machine is so constructed as to actuate the plate holding cylinder or the blanket cylinder by means of a gear mechanism, an abrupt external force such as frictional force works on the contact surface between the offset master set around the plate holding cylinder and the blanket cylinder at the time of engagement of the teeth of the gears, and a spot of the contact surface subjected to said external force is apt to be smeared with ink somewhat earlier than where it is inked normally (this smearing of ink is hereinafter called `gear-stripe`). And, in fact, occurrence of this `gear-stripe` cannot be prevented by the use of the conventional treating liquids.